1. Field of the Invention
The present invention relates to an overheat protection device used for prevention of overheating of the surface of a movable body, which is heated by a heating device while temperature control is performed on the surface, an overheat protection apparatus, and a temperature control device having a function as a temperature detection device for temperature control in addition to a function of such overheat protection.
2. Description of the Related Art
Conventionally, some electronic devices, electric heaters, heat exchangers and the like (e.g., water heaters and air heating apparatuses) detect the temperature in a predetermined position with a temperature sensor for temperature control. These devices are provided with a temperature control unit to stop energization or combustion when the detected temperature has exceeded a target temperature. However, there is a possibility that such temperature control unit operates abnormally due to a trouble or break in circuit parts of an internal control circuit. Accordingly, a protection device for overheating (overheat protection device) is previously provided in a position to avoid abnormal overheating in addition to the above-described temperature control unit. As a safety measure, when the protection device is activated, a break is caused in a power circuit of the heating device, thereby a serious accident can be prevented.
As such a protection device, a resettable device such as a bimetal switch and an unresettable type thermal fuse using a thermo-sensitive pellet (sensor) of insulating chemical material or fusible alloy which melts at a particular temperature are known. Among the latter devices, generally, a thermal fuse using fusible alloy has a simple structure, and is a low-cost and low-price device.
FIG. 17 is a plan view showing an example of a thermal fuse (hereinbelow, also referred to as a “fusible alloy thermal fuse”) using a fusible alloy. FIG. 18 is a cross-sectional view along a line K—K in FIG. 17. Note that in FIG. 17, some members are cut, and portions that would exist if not cut, or portions hidden with cut parts are indicated with dot and broken lines.
In FIG. 17 and FIG. 18, reference numeral 110 denotes a rectangular insulating substrate of ceramic material such as alumina. A pair of electrodes 104a and 104b, of calcined Ag conductive paste such as Ag paste, AgPd paste or AgPt paste, are formed at both ends of the insulating substrate 110. A fuse element 102 of a fusible alloy which melts in response to ambient temperature is connected, by welding and the like, between the pair of electrodes 104a and 104b, in a state where it bridges the both electrodes, integrally with these electrodes. The surface of the fuse element 102 is covered with flux 106, and the entire flux 106 is covered with an insulating cap 108 of mold member of alumina ceramic or resin, and further, the perimeter of the cap is fix-sealed with seal resin and the like. In accordance with necessity, leads are connected by soldering and the like to the pair of electrodes 104a and 104b, and the fuse is provided as a thermal fuse.
FIG. 19 is a longitudinal cross-sectional view showing an example of a so-called axial type (cylindrical) fusible alloy thermal fuse. In the fusible alloy thermal fuse, a pair of leads 114a and 114b with round ends and cross section are provided such that the ends having an electrode function are opposed to each other. The opposed ends of the pair of leads 114a and 114b are fixed by welding and the like to both ends of a fuse element 112 of a low-fusion point alloy having round ends and cross section, and covered with flux 116. Further, the fuse is inserted into a cylindrical insulating case 120 of alumina ceramic and the like, and openings at both ends of the insulating case 120 are sealed with insulating seal material 118 of epoxy resin and the like.
In the fusible alloy thermal fuse, when the temperature of the thermal fuse itself has exceeded a predetermined temperature as an abnormal temperature in response to thermal conduction, a convection current and radiation from a subject of detection, the fuse element 102 or 112 of fusible alloy melts then the opposed ends of the pair of electrodes 104a and 104b or the leads 114a and 114b are electrically isolated, thus a break is caused in a power circuit of a heating device, thereby a serious accident can be prevented.
However, in a case where the temperature of the subject of detection rapidly rises, the temperature of the thermal fuse is greatly different from that of the subject of detection. In such a case, an influence on a user and peripheral devices can be prevented, but there is a possibility that the function of thermally protecting the subject of detection and ensuring sufficient safety cannot be performed. Further, in a case where the subject of detection is a movable body such as a rotary member, since it is generally impossible to bring the thermal fuse into direct contact with the subject of detection, the thermal fuse is provided with an interval from the subject of detection. In this case, there is no thermal conduction from the subject of detection and the thermal response of the thermal fuse is not excellent. Accordingly, in some cases, when the thermal fuse is actuated, the temperature of the subject of detection has already reached a temperature to cause thermal damage.
A particular example is a rotary heating body (heating roller, a heating belt and the like) of a fixing apparatus incorporated in an electrophotographic apparatus such as a copier or a printer. In such a fixing apparatus, an unfixed toner image is heated and pressurized by the rotary heating body of the fixing apparatus, thereby toner is fuse-fixed. In recent years, in the electrophotographic apparatus, further reduction of heating time (warm-up time) between switch-on and fixing-possible time (improvement in instant startablity) is needed. For this purpose, there is a trend of heating upon start with higher heating energy in comparison with the thermal capacity of the rotary heating body. In such a case, the problem is noticeable in the response of the thermal fuse when the temperature of the rotary heating body as a subject of detection is greatly different from that of the thermal fuse.